Perfectly imperfect: a review of chemical tools for exciton engineering in single-walled carbon nanotubes

Single-walled carbon nanotubes (SWCNTs) have revealed extraordinary potential for photonics. However, the scarcity of convenient methods to control chirality and inherently low quantum yield hamper their implementation in the field of optics. It has been recently discovered that the introduction of a slight amount of disorder to the otherwise perfect nanocarbon lattice can be surprisingly beneficial. Covalent addition of oxygen species or alkyl/aryl groups to the SWCNT side-wall creates local potential wells for trapping mobile excitons. As a consequence, the light emitted from SWCNTs is much brighter and approaches quantum yields of up to 20%. Furthermore, new redshifted signatures emerge in the spectra, which extends their application capabilities. This review summarizes the progress on this front from the chemical perspective. A range of currently available tools to create such defects is provided. To gain appropriate insight, a selection of inorganic and organic chemical reactions, which work on the principles of oxidation and alkylation/arylation, respectively, is analyzed herein. Furthermore, the effect of the structure of the grafted functional group on the optical properties is considered in detail. It is supported by all data extracted from the literature regarding modification of (6,5) SWCNTs, being the most representative example due to their widespread use. This summary serves as guidance for new scientists entering this promising area of research. Finally, the contribution is concluded with a critical comparison of the methodologies and an overview of the future shape of this field, which indicates which niches should be taken care of first.